President Trump on Monday night threw his full support behind Mitt Romney’s bid to be the next U.S. senator from Utah, writing on Twitter that he would be a “worthy successor” to retiring Sen. Orrin G. Hatch (R-Utah).

The endorsement is significant because Trump had previously encouraged Hatch to seek another term, and Romney had been among the most vocal Republican critics of Trump’s presidential aspirations, once calling him “a phony” and “a fraud.”

MittRomney has announced he is running for the Senate from the wonderful State of Utah. He will make a great Senator and worthy successor to @OrrinHatch, and has my full support and endorsement!” Trump wrote on Twitter on Monday night after returning to Washington from a long weekend in Florida at his Mar-a-Lago estate.

Shortly after Trump's tweet, Romney wrote on Twitter: “Thank you Mr. President for the support. I hope that over the course of the campaign I also earn the support and endorsement of the people of Utah.”

Romney on Friday announced his long-expected Senate bid in his adopted state of Utah with a low-key rollout intended to duck roiling Republican divisions and avoid signaling that he will play the role of foil to Trump.

“Utah’s economic and political success is a model for our nation,” Romney said in a written announcement distributed Friday morning. “I am ready to fight for this great state and advocate for solutions that improve the lives of Utahns.”

In his announcement, and in a video, Romney, 70, touted his leadership of the 2002 Olympic Games in Salt Lake City, his tenure as Massachusetts governor, his degree from Brigham Young University and his 24 grandchildren. He made no mention of Trump.

A Salt Lake Tribune poll last month showed Romney with the support of 85 percent of Utah Republicans — and 18 percent of Democrats.

Despite his harsh criticism of Trump during the 2016 election cycle, Trump considered Romney for his secretary of state but passed him over for Rex Tillerson.

In a March 2016 speech, Romney said Trump’s “promises are as worthless as a degree from Trump University. He's playing members of the American public for suckers: He gets a free ride to the White House, and all we get is a lousy hat.”

Former White House chief strategist Stephen K. Bannon said in recent months that he would like to draft a populist candidate to oppose Romney in the Utah GOP primary race, but his falling out with Trump over journalist Michael Wolff’s book “Fire and Fury” stalled that project. Several little-known Republicans have announced Senate campaigns ahead of Romney.

Robert Costa contributed to this report.